Is the tank isolated against heat radiating from engine and muffler? I allways cover my tanks (metal) with balsa and silver mylar.
When fuel is slowly getting heated during flight, viscosity goes down and causes the engine to go rich.
But, it can also be bad plumbing which I doubt as it's from Kaz.

Lauri

I had the same issue when someone suggested I make the vent from a little larger than 1/8"
tubing. A mistake.

As previously suggested, try making the vent smaller. You can experiment easily at the field with fuel Tubing and brass tubing inserts to see if that's it before rebuilding a tank.

 Curt

I have found that pressure doesn't always detect leaks, especially in our tanks, where almost all end caps are fastened (soldiered or glued) over the main tank....Suction (use a fueling syringe) reveals a little more.
More annoying, is the possibility that the uniflow tube has an internal tear....not visible unless taken apart, I believe. I wish Brett of someone would explain how to detect this without taking it out of the tank....

Good Luck

Hi guys I'm running a pa 61 with a rear muffler using a kaz minato carbon fibre clank tank ,the problem I'm having is it starts off with a nice two four break but Slowly richens up though the pattern by the time I get to the overheads it's a four stroke setting . Any ideas how to fix it.
Setup ventri .211
          Plug thunderbolt big bore
          Fuel 10sny/10castor/10nitro
          No muffler pressure
Cheers Steve

     OY!  A.211 venturi with no spigot?  That's rather large, FAR bigger then I ever ran. I used a #5 drill (.205) with a 1/16x.030 spigot. With a spigot, maybe a .211/#3, but without, wow. You didn't mention a diffuser (air filter or pantyhose), that might give you a chance with that large a venturi and a flush outlet. I think it comes with a #12 venturi (.189) which normally gives very good power and will provide much better fuel suction.

    However, an excessively-large venturi  normally causes it to go lean, not rich. This is not the effect of a uniflow leak, which usually turns it into a suction tank, nor the venting (which is moot aside from windy weather). It *does* sound like the fuel getting heated but you did most of what it needed to combat that. Anything wrong will be greatly exaggerated with the large venturi, but going to a normal venturi will only reduce the issue, not solve it.

    Other than restoring a normal setup with a normal-sized venturi (like a #7 or 8 with no spigot, and maybe #5 with a spigot and diffuser), the only thing I can think is that there is a heat-variable leak in the intake area somewhere. For example, the spraybar is leaking air around the screws and nuts holding it in that clears itself up as the engine temperature changes.  This is pretty common if you do not seal your spraybar to the case with sealant, although usually that causes instability on the ground more than in the air. At least make sure to verify that the nut holding the spraybar and the collet are *tight*.

    Maybe also an air leak around the venturi itself. The PA series is somewhat more prone to that than others because the compression of the o-ring around the venturi and holding it on the back makes it want to tilt in the bore. We usually selected for venturis with the smallest clearance fore/aft and it's important to use the stock *black* o-rings, or the venturis with the undercut if you use the red silicone types. I have seen a lot of people try a "pretty close" o-ring from the hardware store that was too thick to seal properly, or tended to squirt out from under the flange under pressure. The same things happened with the red/orange o-ring, until the bore was undercut to permit proper clearance. Too thick also exacerbates the tendency of the venturi to tilt in the bore.

     Any of those could be it, or something else. This is a strange case, any time I saw this it was the fuel heating up.

       Brett

Remember that the natural operation of a uniflo tank is to richen as the volume of air in the tank gets larger.  Unless you are really rich at the end of the flight, you might have a fairly nominal operation.

    Mine doesn't do that, it runs the same, and that is certainly the purpose of having uniflow.

    Brett

Hi guys I'm running a pa 61 with a rear muffler using a kaz minato carbon fibre clank tank ,the problem I'm having is it starts off with a nice two four break but Slowly richens up though the pattern by the time I get to the overheads it's a four stroke setting . Any ideas how to fix it.
Setup ventri .211
          Plug thunderbolt big bore
          Fuel 10sny/10castor/10nitro
          No muffler pressure
Cheers Steve

Hi Steve

The venturie  is  way  too large, and if someone drilled it out to that size , that is a large problem as it is ruined and is not a venturi any longer, you should have one  with a  # 12  drill size,  however bad that is  it does not  seem to be the cause of this problem
Seems like the first thing to do is try to seal the heat from the tank,  I have had  so many people that had either header or muffler touching, just barely the tank, or extremely close, it would also help to see a photo of the  front end, and how the airflow  get thru.
Double check that your tank is not being too close to the muffler,   I have had people swear it not touching, then I showed them the tiny wear marks on the header or muffler where it was.
Also check that the  needle is not  touching  the fuse  sides, that will cause  rich or lean runs,
Heat  in the engine is another thing to look at,  make sure you get air over the engine  and  muffler  BEFORE  any air  exits, so this means  NO vents on the  cowling before that end of the  muffler

I think you have enough to try  now,  so please  let us know  how it goes

Randy

   

     This is pretty common if you do not seal your spraybar to the case with sealant, although usually that causes instability on the ground more than in the air.
  

Hey Brett, what type of sealant do you use for this ?